Chemistry 240
Summer 2001

Today we'll find that resonance is very important in understanding both
the structure and the reactions of aromatic compounds. First, let's take
a look at the structural representations which distinguish aromatic compounds
from those that aren't aromatic.

The most commonly encountered aromatic compound is benzene. The usual
structural representation for benzene is a six carbon ring (represented
by a hexagon) which includes three double bonds. Each of the carbons represented
by a corner is also bonded to one other atom. In benzene itself, these
atoms are hydrogens. The double bonds are separated by single bonds so
we recognize the arrangement as involving conjugated double bonds. An alternative
symbol uses a circle inside the hexagon to represent the six pi electrons.
Each of these symbols has good and bad features. We'll use the three double
bond symbol simply because it is also routinely used in the text.

Keep in mind that if the hexagon contains neither the three double bonds
nor the circle, the compound is not aromatic. It is simply cyclohexane
and there are two hydrogens on each carbon atom. This is easy to mistake
when hurrying, so be careful when you are intepreting any structural formulas
which include hexagons.

The structure with three double bonds was proposed
by Kekule as an attempt to explain how a molecule whose molecular formula
was C6H6 could be built out of carbons which make
four bonds. The ring and the three double bonds fit the molecular formula,
but the structure doesn't explain the chemical behavior of benzene at all
well. Each of the double bonds would be expected to show the characteristic
behavior of an alkene and undergo addition reactions, but this is not how
benzene reacts.

In particular, we would expect a carbon-carbon double bond to react
quickly with bromine to make a dibromo compound. This is what alkenes do
very readily, and in fact it is a useful test for alkenes in the laboratory.
Benzene does not react with bromine unless a very bright light or a strong
catalyst is used, and then the reaction is not an addition reaction. We
conclude that there is something quite unusual about the double bonds in
benzene.

Kekule (thinking about this problem before bonds were understood as
pairs of electrons) suggested that there are two forms of benzene which
differ in the locations of the double bonds. His idea was that these were
in rapid equilibrium, so rapid that there was never a fixed location for
the double bond. One could say that an approaching bromine molecule could
not "find" a double bond to react with.

There were several other structures proposed for
benzene, but a much more satisfactory approach became possible when we
began to understand that covalent bonds consist of pairs of electrons shared
between atoms. The difference between the two structures Kekule envisioned
(called Kekule structures) is only the difference between the locations
of three pairs of electrons. This is exactly the type of situation where
resonance must be involved. The hybrid or "average" of the two Kekule structures
has one sigma bond and one-half of a pi bond between each two carbon atoms.
Thus each carbon is joined to each of its neighbors by a one-and-half bond.
Each bond in the benzene ring has the same number of electrons and is the
same length. This picture is in complete accord with experiments which
show that all carbon-carbon bonds in benzene are the same length, with
no hint of shorter (double) or longer (single) bonds. It also helps explain
why benzene does not undergo addition reactions: there are no simple pi
bonds.

Recall that resonance has another important feature: when resonance
is involved, the real structure is more stable than we would expect from
any of the structures we write using the one line = two electrons symbolism.
This extra lowering of energy, which for benzene is about one-third as
much as making a typical covalent bond, is quite important in the reactions
of benzene and other aromatic compounds. As we will see, reactions of the
benzene ring almost always result in products which in which the benzene
ring persists -- an outcome of its stability.

When resonance theory was first applied to understanding
the structure of benzene, the key feature seemed to be a resonance hybrid
of ring structures containing alternating single and double bonds. This
immediately led to attempts to make and study compounds like cyclooctatetraene
and cyclobutane. These compounds also have ring structures with alternating
single and double bonds.

Cyclooctatetraene has been made, but it does not posess the properties
of extra stability and resistance to addition reactions which distinquish
aromatic compounds. It readily adds bromine, for example. Cyclobutadiene
is extremely unstable -- one cyclobutadiene molecule reacts with another
cyclobutadiene molecule instantaneously even at very low temperatures --
so it certainly does not act like an aromatic molecule and it has been
called "antiaromatic" as a result.

It seems that there is more to being aromatic than simply a ring with
alternating single and double bonds. After considerable development of
the underlying theory, the pattern which has emerged is that aromatic characteristics
are only expected when there is a ring of pi electrons in which the number
of pi electrons is equal to 4n + 2 (where n is an integer,
0, 1, 2, etc.). (This is known as the Huckel rule after its discoverer.)
We can check this against the compounds we have considered so far: Benzene
has 6 pi electrons (two for each pi bond) which is the number we get from
4n + 2 if n = 1. Cyclooctatetraene has 8 pi electrons, and there is no
integer "n" which will make 4n + 2 = 8. Cyclobutadiene has 4 pi electrons
and also doesn't fit 4n + 2. There are many other examples which support
Huckel's rule.

It is important to be sure that the ring of alternating single and double
bonds is complete. If there is an sp3 hybridized carbon in the
ring, the conditions for aromatic character are not present, and we do
not worry about checking for 4n + 2. Here's an example:

Another way to see this is to look at the p orbitals which combine to
make the pi bonds. If these p orbitals combine to form an uninterrupted
ring as is the case in benzene, then we can go ahead to use Huckel's rule
to check for the proper number of pi electrons for aromatic character.
If the ring of p orbitals is broken by a CH2 (group or another
tetrahedral carbon) with no p orbital, then the compound cannot be aromatic
and we need not try to apply Huckel's rule.

The p orbitals which make up the unbroken p orbital ring can be associated
with other atoms than carbon. Two examples are furan and pyrrole, in which
two of the six electrons needed come formally from unshared electron pairs
on oxygen.

Such an unshared pair can also come from a carbon atom, which will have
to have a negative charge. An example of this is the cyclopentadienide
ion which can be made by treating cyclopentadiene with a moderately strong
base. Cyclopentadienide ion is sufficiently stabilized by its aromatic
character that cyclopentadiene (its conjugate acid) has a pKa
of 16, close to that of water. Cyclopentadiene is a remarkably strong acid
for a hydrocarbon because its conjugate base has the extra stability of
an aromatic compound.

Extraordinarily stable cations can also be made if their structures
are aromatic. Here are two:

Notice that here the formally positively charged carbon atoms are sp2
hybridized and have an empty p orbital which completes the cyclic arrangement
of p orbitals.

Let's finish up today by looking at the general
mechanism for the characteristic reactions of aromatic compounds -- electrophilic
aromatic substitution. The most important characteristics of these reactions
follow directly from the stability of the aromatic ring. First, these reactions
are typically catalyzed by strong electrophilic (Lewis acidic) catalysts
like H2SO4, AlCl3, and FeCl3
which are required to overcome the stability of the aromatic ring. Second,
these are substitution reactions since addition reactions would interrupt
the p orbital ring and destroy the aromatic stability.

Even though the outcome of the attack of electrophiles on benzene is
substitution rather than addition, the first step is the same as in electrophilic
addition to alkenes -- attack of the electrophile on a pi bond and the
formation of a new sigma bond between a carbon atom and the electrophile.
The carbocation which is formed undergoes loss of the H+ from
the carbon which was attacked. The electrons from the C-H bond are returned
to the aromatic pi electron ring and aromatic stability is restored.

Notice that the intermediate here is a carbocation, but it is not aromatic.
The carbon bearing the hydrogen and the electrophile is sp3
hybridized and has no p orbital to contribute to a cyclic p orbital system.
The carbocation intermediate is somewhat resonance stabilized, though,
by a resonance arrangement which is very similar to the one we saw in the
addition of electrophiles to conjugated dienes.

This intermediate is a carbocation, and as we will see next time, its
stability is important in determining how fast the reaction goes and (in
benzene rings which bear substituents at one of the carbons) where the
electrophile attacks. The key thing to recognize now is that the positive
charge and the corresponding carbocation characteristics only appear
at positions ortho and para relative to the point at which
the electrophile attack. (Nomenclature is treated in Sec 6.3 of Atkins
& Carey.) This will turn out to be quite important, so verify this
for yourself.